So-called downsizing is used to reduce fuel consumption of a four-stroke gasoline engine, i.e., reducing displacement while maintaining the same performance data, typically the torque and/or the power. In order not to affect the power, either the maximum rotational speed or the relative air charge of the cylinders is increased with the aid of a crankshaft-driven compressor or an exhaust gas turbocharger.
In the latter case, torque shortcomings are known to occur in the lower rotational speed range. A compromise is usually necessary to adapt a turbo machine to the engine because the torque in the lower rotational speed range differs from that of a naturally aspirated engine of a comparable power class. When an air flow rate during operation has reached a certain level, which depends on the design of the turbo machine, enough charging pressure is built up to raise the torque to the level of the naturally aspirated engine.
This torque shortcoming was to be counteracted while still meeting the other target values, such as low emissions and engine smoothness. Development and research work so far have shown that an inexpensive approach to the aforementioned torque shortcoming is achievable by so-called scavenging.
In the scavenging method, which is a purging method, it is provided that, depending on the number of cylinders and the design of the intake manifold, the inlet system and the valve gear, there is a time interval within the crankshaft angle of 720° of the combustion cycle during which the exhaust gas backpressure is lower than the intake manifold pressure. If the intake valves and exhaust valves are opened simultaneously during this time interval, which is referred to as valve overlap, there is a flow of fresh air through the combustion chamber of the cylinder. This flow purges the remaining exhaust gases out of the combustion chamber, thereby increasing the air charge. The intended increase in torque is one result of this measure. This also significantly improves the dynamics of the internal combustion engine.
One disadvantage of the scavenging effect is that some of the fresh air flowing in through the intake valve goes directly to the exhaust gas system without having been involved in the combustion. This has consequences for exhaust gas purification, e.g., using a three-way catalytic converter, so that the resulting conversion is only partial. If, instead of fresh air, a stoichiometric mixture is available for through-flow, then a mixture of fresh air, vaporized fuel and exhaust gases from combustion is present in the exhaust gas system. This mixture of different gases is not completely converted by the catalytic converter. In the case of a stoichiometric conversion, there is also an exothermic reaction, which may severely damage the catalytic converter or at least greatly shorten its lifetime.
One approach to this challenge is to combine scavenging with so-called direct fuel injection. This technology allows an injection strategy, which prevents unburned fuel mixed with air from being conveyed into the exhaust gas system. However, use of the systems known from the related art in the aforementioned combination causes increased costs. Direct gasoline injection or BDE technology usually requires high-quality components for high-pressure injection, typically high-pressure pumps, high-pressure lines, pressure regulators, fuel injectors having narrow tolerances, etc.
An inexpensive alternative to BDE technology would be the use of traditional intake manifold injection (SRE), although in its present form it is unable to prevent transport of unburned hydrocarbons.
In the standard case, fuel is injected into the intake port in intake manifold injection technology (later referred to as SRE technology) to provide a displacement long before the opening of the intake valve, so that a wall film is formed in the intake port. This wall film is torn away the next time the intake valve opens. This achieves, among other things, a homogenization of the fuel-air mixture, resulting in favorable properties. Emissions are diminished and the catalytic converter's burden is reduced. This makes it possible to meet prescribed exhaust gas standards.
Due to the simultaneous use of upstream injection and the scavenging effect, with the SRE technology there is the risk that a mixture of exhaust gases, fresh air and unburned vaporized hydrocarbons might enter the exhaust gas system directly, resulting in increased emissions.
Various approaches to solving these SRE-specific problems have been published so far and are mentioned here as examples.
Japanese Patent No. JP 2005/083285 relates to a turbocharged gasoline engine and its valve control times. The valve system used here includes two intake valves and two exhaust valves, each having its own intake port, for each cylinder of the engine. These intake valves are opened with a time offset per cylinder, for example, with the aid of a variable offset between the intake cams. This time offset is determined by the design and is consequently not variable. Intake and exhaust valve overlap is required to be able to employ the scavenging effect. Japanese Patent No. JP 2005/083285 therefore describes a system, for example a so-called “variable valve control,” which makes it possible to vary the opening times on the intake side during engine operation so that the overlap of the intake and exhaust valves is freely selectable. With the aid of a single injector, fuel is injected into the intake port, where there is also a first intake valve. This first intake valve opens later than a second intake valve, so that the first intake valve and the exhaust valves do not overlap. The scavenging effect is thus applied on the side of the second intake valve when open states of the second intake valve and of one exhaust valve overlap, and fuel is injected on the side of the first intake valve with upstream displacement without any risk that the fuel will then enter the exhaust gas system in unburned form. Thanks to this configuration, the aforementioned scavenging effect may also be used for the engine without causing increased hydrocarbon emissions.
Japanese Patent No. JP 2005/180285 describes a similar system relating to a control unit for a supercharged internal combustion engine using a turbocharger. Here again, an offset is provided between the cams for activation of the two intake valves, but an injector is assigned to each intake port, so that each cylinder has two injectors. Variable valve control is also used for adjusting a valve overlap on the intake side. If the scavenging effect is to be used, fuel is injected upstream only on the side without valve overlap.